The proposal investigates the role of novel actin-binding proteins (espins) in the development and maintenance of hair bundle structure. The espins are an intriguing new family of Ca2+-insensitive actin-binding and bundling proteins found so far only in specialized cells such as intestinal brush border, Sertoli and hair cells. Family members differ in their N-termini, suggesting specialization of function. The proposed experiments will characterize actin-bundling of the isoforms in vitro, examine their effects on the actin cytoskeleton when transiently expressed in heterologous cell models, and look for molecular partners using the yeast 2-hybrid screen. Light and EM immunocytochemistry will be done to determine intracellular localization and how it changes with development. Finally, two genetic approaches will be taken to function: the anatomical and physiological effects of knocking out the espins will be examined, and the possibility that the jerker mouse mutant is an espin mutant will be investigated.